Wireless communication systems keep evolving to meet the needs for providing continuous and faster access to a data network. In order to meet these needs, wireless communication systems may use multiple carriers for the transmission of data. A wireless communication system that uses multiple carriers for the transmission of data may be referred to as a multi-carrier system. The use of multiple carriers is expanding in both cellular and non-cellular wireless systems.
A multi-carrier system may increase the bandwidth available in a wireless communication system according to a multiple of how many carriers are made available. For instance, a dual-carrier system may double the bandwidth when compared to a single carrier system and a tri-carrier system will triple the bandwidth when compared to a single carrier system. In addition to this throughput gain, diversity and joint scheduling gains may also be expected. This may result in improving the quality of service (QoS) for end users. Further, multiple carriers may be used in combination with multiple-input multiple-output (MIMO).
In the current third generation partnership project (3GPP) release 8 (R8) and release 9 (R9) specifications for single carrier long term evolution (LTE), the wireless transmit/receive unit (WTRU) ensures that its radio connection to the system (i.e., the radio resource control (RRC) connection), once established, is maintained. The WTRU performs radio link monitoring and, upon detection of physical layer problems, increases its monitoring activity until it recovers from the physical layer problems. If the problems persist and conditions to recover from the physical layer problems cannot be met, the WTRU assumes a radio link failure (RLF). Upon RLF, the WTRU performs a recovery procedure, (e.g., an RRC connection re-establishment).
More specifically, radio link monitoring in currently specified LTE systems (with one carrier) is described in several 3GPP documents. A WTRU monitors the downlink (DL) radio quality against thresholds (Qout and Qin) every frame or every discontinuous reception (DRX) period, depending on whether or not DRX is activated. The physical layer indicates in-synch or out-of-synch to higher layers, (e.g., an RRC layer), when the radio link quality is better than Qin or worse than Qout (respectively). The time periods for this evaluation depend on whether DRX is utilized or not. The radio link quality is measured in terms of the block error rate of a hypothetical physical downlink control channel (PDCCH) transmission, taking into account the physical control format indicator channel (PCFICH) errors with a certain assumed transmission configuration.
Upon the reception of a predefined number (e.g., N310) of consecutive out-of-synch indications, the RRC detects that a physical layer problem has occurred and starts a recovery timer (e.g., T310). If the recovery timer expires before a predefined number (e.g., N311) of consecutive in-synch indications are received, (i.e., the “recovery of a physical layer problem”), the RRC determines that RLF has occurred and either initiates a connection re-establishment procedure or the release of the RRC connection, depending on whether access stratum (AS) security has been activated or not. While the recovery timer is running, the evaluation period radio link quality evaluation is as if DRX is not used, regardless of whether it is being configured or not. This implies that the WTRU turns on its receiver for the purpose of evaluating radio link quality, even if no reception of data is expected as per DRX rules.
In addition, the RRC may also determine that RLF has occurred upon an indication from the medium access control (MAC) of a random access problem, or an indication from the radio link control (RLC) that the maximum number of retransmissions has been reached.
To further improve achievable throughput and coverage of LTE-based radio access systems, and to meet the international mobile telecommunications-advanced (IMT-Advanced) requirements of 1 Gbps and 500 Mbps in the DL and uplink (UL) directions respectively, LTE-advanced (LTE-A) is currently under study in the 3GPP standardization body. One improvement proposed for LTE-A is the carrier aggregation and support of flexible bandwidth arrangement feature. One motivation is to allow DL and UL transmission bandwidths to exceed 20 MHz in release 8 (R8) LTE, (e.g., 40 MHz to 100 MHz).
When the WTRU is operating on multiple carriers, the criteria for radio link monitoring and the actions to take upon detection of a radio link problem need to be redefined, as compared to a single-carrier case. This is because of the possibility of different radio conditions over different component carriers and the modifications that need to be made to the control channel structure and related procedures compared to the single-carrier case.